RFR: Add remaining unused free space to 'used' counter in free list

[Roman Kennke]

I noticed that when a program allocates many objects that are slightly
larger than half a region, we would continuously run into full GC. The
reason is that when we skip to next region for allocation, we did not
count the remaining unused free space as 'used', and thus barely
reported half of heap remaining when running OOM. Oops.

Fixed in ShenandoahFreeList by adding last-current-region's remaining
free() to the free-lists used.

Ok?

http://cr.openjdk.java.net/~rkennke/fixused/webrev.00/

Roman